This invention is directed to microemulsions of gel-free polymers, and to a method for making polysiloxane emulsions using what is commonly known as emulsion polymerization. Microemulsions are produced from a mixture of a siloxane oligomer, a hydrolyzable water-soluble alkoxysilane, a cationic or anionic surfactant, a nonionic surfactant, a catalyst, and water. Silicon containing reactants react in the presence of water and surfactants to form polysiloxane emulsions. By using our method, it is feasible to produce microemulsions of gel-free polymers.
Our invention is an improvement on methods described in European Patent Application 0 459 500 (EP 459500), published Dec. 4, 1991, assigned to the assignee of our invention. While EP 459500 teaches similar techniques for making microemulsions, it does not teach how to avoid gelation of non-linear siloxane polymers.
Polysiloxane emulsions are categorized by the size of the polysiloxane particles and the appearance of the emulsion. The art recognizes three categories of silicone emulsions, (i) standard emulsions, (ii) fine emulsions, and (iii) microemulsions. Silicone standard emulsions have a large particle size greater than 300 nanometers and appear to the human eye to be opaque and impenetrable to light. Silicone standard emulsions have an intense white appearance. Silicone fine emulsions have a smaller particle size from 140-300 nanometers and visually are slightly opaque to very slightly translucent. Fine emulsions transmit light but with distortion. Silicone microemulsions have a particle size less than 140 nanometers and visually appear translucent to transparent and transmit light without distortion. Microemulsions are most desired due to smaller particle size, higher stability, and translucent to transparent appearance.
Emulsions of polysiloxanes in water can be made by mechanically or by emulsion polymerization. Mechanically means taking the preformed polysiloxane and using mechanical apparatus such as a homogenizer or vigorous agitator to emulsify the siloxanes in water. A surfactant can be added to the polysiloxane or water to aid the emulsification process.
Emulsion polymerization to which our invention pertains entails combining silicon containing reactants, surfactants, polymerization catalyst, and water. The mixture is stirred and the silicon containing reactants are allowed to polymerize until a microemulsion is formed. Alkoxysilanes, cyclic siloxanes, and combinations of alkoxysilanes and cyclic siloxanes can be used as reactants to form the microemulsion.
While techniques in EP 459500 have been largely successful in producing suitable microemulsions of linear siloxane polymers, they do not provide for avoidance of polymer gelation when production of a non-linear siloxane polymer microemulsion is the desired product, which is the essence and contribution of our invention.